The decay of chlorophyll (Chl) fluorescence of etiochloroplasts isolated in various stage of greening of cucumber cotyledons was analysed in order to get structural information on a photosynthetic apparatus. Two model decays, multiexponential and stretched exponential, were applied in the analysis. The quality of fit in these two models was different in various stages of chloroplast greening. The two-exponent model did not provide a good fit at early greening stages. To improve the fit it was necessary to introduce an additional third component which became very low at later stages. However, chloroplasts in the early stage of greening could also be described by a stretched exponential with parameters indicating rather planar (two-dimensional) arrangement of donor and acceptor molecules. The chloroplasts treated by DCMU and/or photooxidized by strong irradiance exhibit a similar character of fractal decay as untreated samples but in the multiexponential model the exact values of lifetimes and amplitudes of components vary. This suggests that the structure of investigated system does not dramatically change as a result of these two types of treatment. and A. Kowalczyk, A. Waloszek, D. Frąckowiak.
Effects of high irradiance (HI) treatment on chloroplast pigment bleaching and on low temperature excitation and emission fluorescence spectra were studied in thylakoids isolated from 3- or 24-h greening cucumber cotyledons of etiolated seedlings. Irradiation of thylakoids isolated from 3-h greening seedlings with HI (approx. 3500 pmol m'^ s'*) caused chlorophyll (Chl) and carotenoid (Car) breakdown with the initial rates 3.0 and 4.2 mg m’^ s'*, respectively, whereas these values were 1.3 and 0.8 mg m-^ s'^ respectively, in thylakoids isolated from 24-h greening cotyledons. The photosusceptibility decreased in the sequence: protochlorophyll(ide), P-carotene, xanthophylls, Chl a, Chl b. In control seedlings the ratio E472/E437 or E486/E437 was 1.7-1.8-fold higher at the later phase than at the early phase of greening whereas the ratio F730/F684 reached approx. 0.40 or approx. 1.65 after 3- or 24-h greemng, respectively. Treatment of isolated thylakoids with high photon flux density caused a relative increase of the 472 and 486 nm band intensities in the excitation fluorescence spectrum of Chl, whereas the ratio F730/F684 increased only at the later phase investigated.
The character of interaction between carotenoids (Cars) and chlorophylls (Chls) in thylakoids isolated from cucumber cotyledons at three stages of greening (3, 6, and 24 h of irradiation with 120 µmol m-2 s-1) was studied. The shapes of the steady state photoacoustic spectra were changed with the change in time of greening and with the frequency of radiation modulation. The shapes show that changes not only in the contents of various pigments but also in pigment interactions with surrounding occur and that processes of thermal deactivation characterised by different kinetics take place. Slow processes of thermal deactivation are in most cases due to deactivation of triplet states. Long living triplet states are very often engaged in photochemical reactions that can destroy the tissue. Analysis of the time-resolved photothermal spectra shows that at later stage of greening, the chlorophyll (Chl) molecules are better shielded against photo-destruction because Cars more efficiently quench their triplet states. The yield of formation of the pigment triplet states measured by the time resolved photothermal method, always at the same energy absorbed by pigment mixture, declined during sample greening. The decay time of the slow component of pigment thermal deactivation, due predominantly to deactivation of the triplet state of Chl, decreases with the increase of time of greening from 6.2 µs for the 3-h sample to 1.5 µs for the 24 h sample. The energy taken by Cars from Chls is dissipated into heat, therefore the steady state and quick thermal deactivation values increased during the greening process. The Cars/Chls ratio in the thylakoids decreased during greening approximately 2 fold. Hence at a later phase of greening the Cars can quench the triplet states of Chls more efficiently than at an earlier phase of greening. and A. Waloszek ... [et al.].
Effects of heat pre-treatment (60 oC for 15 min) on the chlorophyll (Chl) bleaching and excitation energy transfer in thylakoids, isolated from the cucumber cotyledons, were studied by anaiysis of the overlapping absorption and the low temperature fluorescence spectra. The Chl forms C678 and C672 appeared to be the most heat-susceptible components, whereas C684 was found to be very heat-stable. Subsequent exposure of the thylakoid suspension to a high photon flux density (ca. 3 500 pmol m‘2 s'i) caused further bleaching of all the tested forms: ca. 8-20 or 2-5 % of chlorophyll was degraded duríng the initial (0-4 min) or later (4-10 min) period of irradiation, respectively. The heat pre-treatment of thylakoids led to the decrease of the 472 and 486 nm bands in Chl fluorescence excitation spectrum and the 730 nm band in the Chl fluorescence emission spectrum. The F730/F685 ratio decreased by ca. 4-, 3- or 2-fold when excited with the wavelengths of 437, 742 or 486 nm, respectively. These ratios increased by 60-90 % after 10 min irradiation with high photon flnx density.